In order to check the existence of ticks with this flag combination (which seemed weird to me), I opened the MT5 screen and clicked on 'View" - "Symbols" - "Ticks", then I requested the tick data in text file format and could confirm that "Buy/Sell" ticks really exist:

Green circles appear when the direction of the transaction is undefined. It is used when the exchange does not transmit the direction of a transaction. In this case, the direction is determined based on the price of the transaction as compared to prices bid and ask. A Buy transaction is that executed at the ask price or above, a Sell transaction is executed at the bid price or lower. The direction is undefined if the price of the transaction is between the bid and the ask.

However like you, I don't know why a transaction can be executed between bid and ask price. This article should interested you, but it doesn't answer this issue, unless I missed it.